Carbon nanotubes (CNTs) are currently being investigated for use as cold electron sources in a variety of applications. These include displays, microwave sources, x-ray tubes, etc. For CNTs to be used as a cold cathode, they must be placed on a conductive surface (conductive substrate or conductive film on a non-conductive substrate). This has led some to place catalysts on the substrate surface and grow the carbon nanotubes in situ using CVD techniques (Kim et al., J. Appl. Phys., 90(5), 2591 (2001)). However, this has several draw-backs. This technique typically grows multi-wall carbon nanotubes (MWNTs). However, MWNTs have poorer field emission quality compared to single-wall carbon nanotubes (SWNTs) (Kurachi et al., “FED with double-walled carbon nanotube emitters,” the 21st International Display Research Conference in Conjunction with the 8th International Display Workshops, Nagoya Congress Center, Nagoya, Japan, Oct. 16–19, 2001, pp. 1237–1240). The substrate is subjected to high temperature, typically above 600° C., limiting the substrates that can be used. Uniformity is difficult to achieve because of the high temperature growth processes required. As a result, the manufacture of cathodes using this process will be very expensive due to the number and complexity of post-processing steps needed to generate a material capable of producing the desired level of field emission.
Other investigations have centered on processes for making CNT cathodes in a separate process, collecting them, and then dispensing them onto a substrate using a variety of techniques (Kim et al., Diamond and Related Materials, 9, 1184 (2000)). This has several advantages over the in situ method described above. First, the fabrication of the CNT material is decoupled from the fabrication of the cathode. This permits choosing the optimal CNT material for the application (single-wall, double-wall, multi-wall, purified, non-purified, etc.). Second, the dispensing process is carried out a relatively low-temperatures, permitting greater flexibility in the choice of substrates. Third, uniform deposition over large area substrates is far more feasible using currently-available, low-cost equipment. Current dispensing processes, however, have their disadvantages. One of these is that the CNT fibers are often dispensed such that they clump together or are imbedded inside another material (Kim et al., “Toward a ridge of carbon nanotube FEDs,” the 21st International Display Research Conference in Conjunction with the 8th International Display Workshops, Nagoya Congress Center, Nagoya, Japan, Oct. 16–19, 2001, pp. 1221–1224). These factors limit the performance of the CNT material. “Activation” processes are often employed after dispensing the CNT material. These processes recover some of the performance of the virgin CNT (Chang et al., U.S. Pat. No. 6,436,221 B1). These “activation” process steps, however, can add cost to the product and may lead to non-uniform performance. Yet another disadvantage of current dispensing techniques is that the dispensed CNT fibers may not have sufficiently good contact to the substrate or the substrate's conductive layer such that this impedes their ability to supply the electrons needed for field emission.
It has been recently found that by mixing CNT material with other nanoparticle materials, the field emission properties of the CNT were improved (Mao et al., U.S. Provisional Application No. 60/417,246, incorporated herein by reference). Because neighboring nanotubes shield the extracted electric fields from each other (Bonard et al., Adv. Mat., 13, 184 (2001)), it is believed that this improvement is a result of induced separation of the CNT material by the nanoparticles. In situations where the CNT fibers are too close, they may electrically screen the applied electric field from each other. By increasing the separation between the fibers, the effective applied field strength at the emission sites is higher.
Many SWNT fibers are semiconducting with a bandgap that is dependent upon the chiral indices (n,m) of the SWNT. Choi et al. (U.S. Pat. No. 6,504,292 B1) teach that, for field emission applications, this bandgap can be overcome by depositing a metal film on CNT fibers that are already attached to a substrate. Choi et al. teach that the CNT fibers are coated after the fibers are grown using CVD techniques. This method has the inherent aforementioned disadvantages of growing CNTs on the substrate. Furthermore, were the CNT fibers to be dispensed onto the substrate and then coated, the problems of separating the CNT fibers for improved emission would still remain.
A method of aligning CNTs is disclosed in U.S. Pat. No. 6,312,303 B1 to Yaniv et al. (incorporated herein by reference), whereby CNTs are aligned by including the CNTs in a host material, aligning the host material (such as liquid crystal material) and the host phase material then aligns the CNTs.